Your circumference is actually 76.65mm. Therefore one full revolution of rotation gives 76.65mm of linear motion.
You are most likely using 1.8* steppers. That means there are 200 steps for a 1:1 in rotation. If you are running 1/16 microstepping, then you have 3200 steps per revolution.
3200 / 76.65 = 41.75 steps/mm
If you somehow are only running 1/8 microstepping, then you have 1600 steps in one rotation.
1600 / 76.65 = 20.87 steps/mm
That would give you a difference of 20.88mm of larger movement. I would check to make sure you are at 1/16 microstepping.